This invention is in the field of nonwoven fabrics and methods of treating the same.
Nonwoven fabrics include those commonly termed spun-bonded fabrics, which are well known and comprise generally fabrics formed by spinning continuous filaments of suitable materials and laying them in web form with the filaments randomly arranged so that portions extend in all directions. The webs are then treated to cause the filaments to bond to each other at their intersections, either by mechanical bonding, fusion or by the use of separate bonding materials. Such fabrics and method for their manufacture are well known and one such method is described in the patent to Kinney U.S. Pat. No. 3,338,992. A further patent describing these materials is the patent to Hartmann U.S. Pat. No. 3,544,854.
As used in this application, the term nonwoven fabric is intended to refer to fabrics of the type having randomly oriented filaments bonded at their intersections by mechanical means, material fusion or separate bonding materials and whether or not the filaments are continuous.
The known nonwoven fabrics are generally considered unsatisfactory for many purposes because of their stiffness or poor drapability.
The compaction of certain types of nonwovens has traditionally been somewhat less than satisfactory in regards to the improvement in softness obtained. This has been particularly true with spun-bonded fabrics of polyester, polyamide, and polyolefins. It has been felt that the problem lies in the fact that the fiber in these fabrics will absorb very little moisture and, therefore, cannot be plasticized or softened by rewetting. Consequently, because of their greater stiffness at the time of processing, relatively high forces are required to buckle the fiber and compact these fabrics. The result has generally been that compaction of spun-bonded fabrics results in a coarse macrocrepe which results in some stiffness reduction but also an undesirable harsh surface quality. This effect is more or less pronounced depending on the fiber denier and basis weight of the material.
Previous attempts to overcome this problem involved efforts to accurately control compaction temperature so as to soften the fabric and make the fibers more pliable and susceptible to compaction. This has been a generally unsatisfactory solution. Even though the fabric may be considerably shrunk in this manner, upon cooling after compaction, the stiffness of the fabric is seldom reduced and in many instances may actually be increased. Too high a temperature is known to have a detrimental effect on softness.
Attempts were also made to reduce the compressive resistance of the fibers by the addition of chemicals known to act as swelling agents. The results were all unproductive, generally because the chemical agents acted as lubricants and hence interferred with the compaction process. In addition, none of the chemicals evaluated produced a significant reduction in the compressive modulus of the material being treated.
Considerable success has been achieved in improving softness of certain types of fabrics by conventional compaction. However, the conventional compaction process acts predominantly on those fibers and fiber segments, oriented in the longitudinal direction of the fabric. Consequently, the reduction in fabric stiffness obtained by this process is mainly limited to the longitudinal direction of the fabric while stiffness in the fabric's transverse direction is reduced only slightly.
Attempts to increase the compressive forces available also centered on the use of antilubricants to increase the friction between the blanket and the material. Mechanical embossing of the web was also evaluated as a means of increasing friction. Additionally, a harder blanket (60 Shore A Durometer va. 50 Shore A Durometer) had a significant effect. The harder blanket produced a finer compaction particularly on the heavy-weight materials.